Method of coating electron emissive cathodes



Oct. 7, 1952 w. P. TooRKs METHOD OF COATING ELECTRON E MISSIVE CATHODES Original Filed May 23 INVENTOR. ilmm P. ToorKS ATTORNEY D atl F Patented Oct. 7, 1952 METHoDor coA'rING eLrzc'raoNl EMIssIvE I Y oA'rHoDEs 4 Y v William P. Toorks, Roxbury, Mass., assigner to Sylvania Electric Products Inc., Salem, Mass., a corporation of Massachusetts Original application May 23, 1945, Serial No.

` '595,309. Divided and this application December 30, 1947, Serial No. 794,519

" v4` claims. (c1. 11n-33.19)

directly upon the proper function of the cathode.V

The number of electrons obtainable from the cathode is greatlyincreased by applyinga coating of electron emissive material thereon; By

applying a relativelythick coating to the cathode its life is prolonged. It is highly important however, that the coating be formulated and applied With particular care to insure the full benefit therefrom. f

It is the general object ofthe invention to increase the life andeiiiciency of the cathode'and particularly its coating.

A specific object of the invention is. to provide an electron emissive cathode-coating which is homogeneous and of maximum density.`

2v Y which steps the liquid solvent is removed. After centrifuging the' ca thodes are removed from the dense mass of emissive'material and `their exteriors smoothed to present an even surface and vafter drying, allmaterial is removed from areas not intended for electron emission thus leaving a densel homogeneous layer of coating material upon the cathode. i To present albetter understanding of the in vention a particular embodiment thereof will now be described in connection with the accompany ing drawings, in which:

Figure 1 is a perspective View, of a' cathode before its emissive material is applied;v

Figure 2 is a view ofla completed cathode; Figure 3 is a cross sectional View of a centrifuge container showing the coating material and the cathode therein; Figure 4 is a top plan view of a centrifuge suitable for processing the cathodes;

`Figure 5 is an enlarged cross sectional -view through a completed cathode;

A further object of the invention is to provide a cathodecoating Which is evenly and completely distributed over the emissive areas of the cathode. A further object of the invention is'to provide a method of applying lthe emissive, coating to cathodes which Will insure maximumperformance and complete uniformity thereof.

Electronic emissive coatings heretofore applied have lacked the required resistance to disintegration and high emissivequality to insure highest possible performance. The coatingprovided by the present invention exhibits several important improved features. It is characterized by a uniformly increased density. Its adherence lto the cathode proper is very secure being'chemiu cally and mechanically perfect. o

When the invention is applied to ay cathode having a mesh screen or the like upon its' emissive area the beneficial results are particularly effective. In such case the active 'material adheres strongly to and covers every portion of the mesh.

The full eifectiveness of the added material held by the mesh is thus obtainable Without-the pos sibility of premature disintegrationand loss of material. A- o The emissive material is applied toy thecathode in a specic manner. Any suitable `@missive material may be used. The material is mixed'in suspension in a volatile liquid and the liquid'is placed in the containers of a centrifuge. The cathodes are immersed in the liquid and centrifuged vfor a Figures 6 and '7 illustrate other types of cathodes to Whichthe invention-may be applied; and

Figure 8 illustratesa cathode forming operation. e A

The invention may be applied to any cathode vand is particularly adapted ,to cathodes having metallic mesh `thereon to `receive the emissive material. One such cathode is illustrated in the drawings and is provided with a body i of suitable metal such as nickel. The body I desirably is tubular in form presenting an aperture S within which a heating element 9 is received and through which the heater lead wires Il! are fed.

The electron emissive section II of the cathode shown in Figures 1 2, l3, and 5 is conned to one end thereof, the remaining portion serving as a support and a conductor for carrying current. The section I I desirably is provided with a section of metallic mesh I2 extendmgfaround theoutside of the cathode and in lcontact therewith. The mesh may be sintered intooontact with the cathode. The cathode may also be provided With iianges i3 adjacent the ends of the gauze I2. The fianges I3 do not constitute a partei" the invention but serve to influence the generaloperation of the tube invvhich the cathode is used,

Any suitable cathode coating material may be used. A satisfactory material consists of approximatelyia 2% solution of nitrocellulose in amyl acetate to which is added a suitable electron emissive material such as a combination of barium, strontium andcalcium carbone-tes. The active material is mixed with the nitrocellulose solution to forma suspension thereof. Barium,

strontium and calcium -carbonates are the mate- 'given period desirably in several Steps between rials which, in their final state, constitute the down in each tube I4 of the centrifuge I5. AnA equal number is placed in each tube to provide the proper balance for operation of the centrifuge. The apertures 8 are plugged to prevent coating material from entering. The plugging.

may be done 'by means of small clean Woode plugs I6.

A quantity of coating is then placed in each tube I4. Suicient material is used to cover the by spraying, contains voids and air holes which lower its efficiency. This condition is overcome inthe present invention by the centri-fuging operation which insures'. that notonly will there be no voids or air holes in the coating but that the material will firmly pack into all the spaces in and formed by the mesh.

Desirably the tubes I4 are provided with screw caps I'I and a nickel gasket I8 to close their bottom ends. Y This-.construction provides for ready removal ofthe cathodes after centrifuging. The caps'rII areremoved and by means of a plunger which substantially fits the tube, the cake I9 containing the cathodes is pushed out through the cathodes to a point a substantial distance' above the gauze I2. The centrifuge is then operated for a, period of substantiallyforty-ve minutes at full speed. The liquid above the partially packed mass -in the tube isthenfsyphoned olf and more coating'material is added to bring theglevel up to the original level. The centrifuge is again run at full speed for approximately thirty minutes and the liquid syphonedoif. The centrifuge is then operated for between three and four hours removing Ythe liquid atintervals when necessary. The times forcentrifuging are given by way of example. Thedesiredresult may be achieved by a suitable period of'treatment depending upon the' conditions-such as the viscosity of the material .and thel speed of the centrifuge.

Approximate values fortest runs made with emissive material of approximately 2 micron particle size following the procedure above-outlined are as follows. In a test in which the centrifuge was operatedat 715'gravities for 5 hours a density of 1.64 was attained; In other tests when operating the centrifuge at 2250 gravities for 5 hours and 22 hours respectively, densities of 2.4 and.2.64 were attained. These are bulk densities, in grams per cc.

The coating material willnow be found to have formed a firm condensed cake in which the ends ofthe cathodes are embedded. By reason of the action of the centrifuge the coating material will be perfectly distributed with a firm even density upon every portionof the exposed portions of the cathodes landthe gauze bands I2. The pressure exerted upon the particlesl of ma.- terial in suspension duringcentrifuging, insures the best possible arrangement of the material and also perfect adhesion ofthe material to the cathode surfaces. By centrifuging in the manner described a very large proportion of .the liquidsinthe suspension are removedandthe active material is thus densiiied substantially to its final volume before drying. Thus the -final condition of the coating is determined by the centri.- fuging operation. In contrast to. this` highly .eiiicient result prior to the present invention the suspension of coating material was painted on the cathode by hand. Such procedure failed to produce the desired result. Lack of uniformity caused many rejected cathodesand. a general spotty porous condition in the coating prevailed due to lack of proper packing oi the.material. This lack of qualityV resulted also from-the fact that when coated by hand the material contained a relatively large percentage of vliquid solvent and temporary binder which when finally removed left the' coating in a-weakened condition containing an amount of active particles Vmuch less than is the case when thetpresent invention is practiced. Also a coating applied b y-hand or top yofthetube. and desirably is placed in a shallow container to hold the cake in shape and each cathode isremoved from the cake by withdrawing While rotating it with the iingers. Before the coatinghasfdried completely it .is smoothed down upon'the mesh.. This may be-.done in any suitable manner. such'as by mounting Ythe cathode ina revolvingspindley andholding` a-wooden or glass spatula against'the surface of the coating and thus scraping the cathode. The coating should be leveled olf uponits surface-so that only 'the' outermostv or.r high. spotsv of the gauze are visible. Insmoothing or leveling-the coating'material asabovedescribed its thickness may be varied to adesired extent byremoving more or less ofthe materialv adhering-to the. emissive portion of the cathode.'A

Thecathodes vare then'set' aside and'allowed to completely or 'partially dry. In connection with the cathode above described all coatingmaterial is then carefully removed except that upon the electron emittingl section. The plugs I6 are removed and any-material'which has seeped past the plugs is cleaned from the walls of the aperture 8f. The cathodes yarethen ready to be assembled in the particularelectronic device for which they are designed. f

Other types of cathodes'may be coated such as-those illustrated in Figures 6 andl 7. The cathode shown in Figure 6 lends itself to the coating method of theinvention. This typeof cathode may or may not be provided with a screen' of gauzeZIlwound` upon the central portionof its cylindrical' body 2l. These cathodes areftreated similarly to the-*cathodes 'I except that `they'are completely-covered by-the cake of coating materialatthe end-of the centrifuging period after which the cathodes. areA removed from the cake preferablyiendwise. The coating material isthen smoothed down uponzthe gauze 20Min the vmanner above described and after drying, the-'cathode iscleaned up by removing allmaterial adhering to its non-emissive portions.

Other'types of cathodesmay also betreated in a similar manner such as the fllamentary cathode shown in Figure v'7.' This cathode. is used in electric discharge lamps such as the hot-cathode fluorescent lamp.

It has been found in use that cathodes pref pared in accordance .with theinvention have exhibited extremely eflicient characteristics and long life.. This is. especially true whenthe cathodes Aare used in the severe servicewherein the cathode is called upon to deliver pulsesv of high instantaneous current density. In such service it hasbeen foundthat an increase inefficiency of as high `as 200 to 900% over'thatexhibited by cathodes prepared inthe conventional manner was obtained. f.

The practice of the :present invention provides ameans for producing a lcoatingof .adesiredden-` sity with a high degree of accuracy. With suspensions containing electron-emitting material of a given particle size, the density of the coating may be controlled to an accuracy of plus or minus 3% over a wide range vof density values. Thisv control is established by the speed of the centrifuge and the duration of the centrifuging period. The speed of the centrifuge more properly should be expressed in gravities which would then apply to a centrifuge of any type and dimensions. This control is maintained independently of all other factors for example, atmospheric conditions, operating temperature, operator technique, impurities in the material and other factors.

The method of the invention can be applied to all types of cathodes including not only those having a smooth surface but also those having irregular or reticulated surfaces of such form that at least some of the reticulations are superb posed upon or masked by other reticulations. Such cathodes cannot be completely filled by the conventional methods of spraying, painting andv cataphoresis. The presentinvention however,

completely fills such cathodes eliminating holes having numerous interstices, comprising the steps of lmmersing the cathode in a coating suspension of electron emissive powder in a volatile liquid containing a small percentage of temporary binder, centrifuging the cathode in the suspension so as 4to compact the powder about and within the interstices of the cathode and thus depositing on the exterior thereof an adherent coating in excess amount, and thereafter cutting the excess coating material from the cathode.

2. The method of coating an external surface of an open-ended hollow cylindrical cathode, including the steps of plugging the open end of the cathode, immersing the cathode ina liquid suspension of a powder of electron emissive material in a volatile liquid containing about 2% of temporary binder relative to the volatile liquid, the extent of immersion being a substantial distance above the surface to be coated, centrifuging the cathode for a long period of time whereby a dense coating of electron-emissive material is tion may be molded into a desired form before it dries or it may be machined to close tolerances after it dries. As shown in Figure 8 a coated and dried cathode 22 is shown in cross-section arranged to be rotated by any desired power means. A tool 23 is mounted or supported in any desired manner and held in cutting relation to the coating material 24 to thus form the coating accurately to a predetermined dimension.

It has been found that coatings prepared according to the invention because of their uniform density require greatly reduced amounts of binder as compared to the conventional coating. It has even been found practical in some instances to use no binder.

Another advantage of the invention is that contamination during the process is reduced to a minimum because the procedure may be carried out in enclosed apparatus thus reducing contamination from the atmosphere.

A cathode coating of finely-divided powders is, of course, porous because of the spaces between the closely-packed particles. 'I'he present invention reduces this porosity by packing the particles still closer together and thereby increasing the bulk density to the figures given earlier in this application. In painting cathodes` by hand, as is generally necessary with mesh cathodes, the bulk density of the coating is extremely nonuniform, and even with sprayed cathodes the coating density is different near the metal base surface than at a distance from the base. My invention produces a uniform density throughout the distance from the base. My invention produces a uniform density throughout the volume of the coating.

In using a centrifuge according to my invention it is desirable to keep the vibration as small as possible, because vibration tends to diffuse the coating particles and hence to counteract the centrifuging action.

What I claim is:

1. The method of coating an external portion of a hollow or cylindrical cathode with an electron emissive powder the portion to be coated compacted about the cathode in excess of the amount finally desired to remain on the cathode, and after centrifuging removing the excess of the densecompacted coating from the cathode by scraping.

3. The method of providing a portion of the exterior of a cathode with an electron emissive surface, which includes the steps of immersing the cathode in a liquid to an extent substantially'beyond that required to cover said exterior portion. said liquid containing a powder of electron emissive material suspended in a volatile liquid containing a small percentage of a suitable temporary binder, centrifuging the cathode for a long period of time whereby a soft but dense coating of electron emissive material is compacted about the cathode in excess of the amount finally desired to remain on the electron emissive portion of the cathode, and after centrifuging, mechanically removing the excess of the dense compacted coating from the cathode while the coating is still soft.

4. The method of providing a dense electron emissive coating on an external mesh-covered region of a cathode which includes the steps of blocking ofi a region of the cathode that is not to be coated, immersing the cathode in a, liquid substantially beyond the extent required to cover the region to be coated, said liquid having in suspension an electron emissive powder, centrifuging the cathode for a long period of time whereby a dense coating of the electron emissive powder is compacted about the cathode in excess of the amount finally. desired to remain on the cathode, and after centrifuging, mechanically removing the excess of the dense compacted coating from the cathode.

WILLIAM P. TOORKS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 934,833 Parker Sept. 21, 1909 1,217,071 Rust Feb. 20, 1917 1,532,533 Harris Apr. '7, 1925 1,939,075 McCulloch Dec. 12, 1933 2,009,232 Hood July 23, 1935 2,119,309 Batchelor v--- May 31, 1938 2,158,981 Collins et al May 16, 1939 2,355,232 Nelson et al Aug. 8, 1944 

1. THE METHOD OF COATING AN EXTERNAL PORTION OF A HOLLOW OR CYLINDRICAL CATHODE WITH AN ELECTRON EMISSIVE POWDER THE PORTION TO BE COATED HAVING NUMEROUS INTERSTICES, COMPRISING THE STEPS OF IMMERSING THE CATHODE IN A COATING SUSPENSION OF ELECTRON EMISSIVE POWDER IN A VOLATILE LIQUID CONTAINING A SMALL PERCENTAGE OF TEMPORARY BINDER, CENTRIFUGING THE CATHODE IN THE SUSPENSION SO AS TO COMPACT THE POWDER ABOUT AND 